Method and apparatus for varying the frequency of an alternating voltage

ABSTRACT

A method and apparatus for the timed or pulsed conversion of analog voltages into step or digitalized voltages wherein the clock frequency pulses are converted into a saw-tooth voltage waveform having a statistically irregular frequency and a random variation of shape, the saw tooth voltage is superimposed onto the analog voltage at the conversion, the frequency of the resultant saw-tooth voltage is non-harmonic to the clock frequency.

United States Patent 1191 Taudt et al. I

[ METHOD AND APPARATUS FOR VARYING THE FREQUENCY OF AN ALTERNATING VOLTAGE Inventors: Heinz Taudt, Kiel; Wolfgang Bruckner, Forstinning, both of Germany Dr. lng. Rudolf Hell GmbH, Kiel, Germany Filed: May 1, 1972 Appl. No.: 248,931

[73] Assignee:

[30] Foreign Application Priority Data May 5,1971

U.S. Cl 328/185, 307/228, 307/260, 328/14, 328/59, 328/156, 340/347 A-D Int. Cl H03b 19/00, H03k 4/10 Field of Search 307/228, 260, 14; 328/34-36, 59, 156, 157, 185-187; 331/78; 340/347 A-D Germany 2122107 Apr. 9, 1974 [56] References Cited UNITED STATES PATENTS 2,845,532 7/1958 Knowlton, Jr. et al. 328/156 x 3,479,496 11/1969 Buesch et al. 307/228 x 3,573,652 4/1971 Charters 331/78 Primary Examiner-Stanley D. Miller, Jr. Attorney, Agent, or Firm-Ernest F. Marmorek [57] ABSTRACT A method and apparatus for the timed or pulsed conversion of analog voltages into step or digitalized voltages wherein the clock frequency pulses are converted into a saw-tooth voltage waveform having a statistically irregular frequency and a random variation of shape, the saw tooth voltage is superimposed onto the analog voltage at the conversion, the frequency of the resultant saw-tooth voltage is non-harmonic to the clock frequency.

6 Claims, 8 Drawing Figures COMPARATOR 20 SAW-TOOTH VOLTAGE SUPER IMPOSED TO THE ANA= LOG VOLTAGE ANALOG VOLTAGE RANDOM GENERATOR ISTORAGE 1sw/ TCHING DEVI CE AMPLIFIER METHOD AND APPARATUS FOR VARYING THE FREQUENCY OF AN ALTERNATING VOLTAGE FIELD OF THE INVENTION The present invention relates to a method for the timed or pulsed conversion of analog voltages into step or digitalized voltages wherein to the analog voltage to be converted an a.c. voltage is superimposed with a frequency which is non-harmonic with respect to the clock frequency.

BACKGROUND OF THE INVENTION In the optical-electrical reproduction of pictures the digital processing method is used increasingly, especially for the correction, contrasting or data storage phases. In this connection it is necessary that the density capacity between the white and black should be subdivided into a gradated scale of grey tints and such gradations mustbe numbered. During scanning of the picture the individual numbers by comparing the scanned values with the grey scale become continuously picked up and registered. During the reproduction process the recording devices are controlled according to such numbers.

In principle, the quality of the reproduction is the better the larger is the number of the degree of shadings constituting the picture. In practice it is desirable that such numbers be kept as small as possible in order that the necessary equipment be kept at a minimum and in order that during the data processing one could get by with the possible smallest binary numbers.

During the reproduction with the help of points having a limited number of fixed density values the danger is present that at picture portions having soft shading, zones of individual shading grades are reproduced and appear as stripes. It is known from German laid open publication No. 1,772,367 the technique of eliminating such defects, according to which onto the analog voltage scanned by the reproduction device preferably a saw tooth-shaped a.c. voltage is superimposed with an amplitude having half the difference of the adjacent step voltages and a frequency which is non-harmonic with respect to the frequency of the scanning, that is, of the reproduction apparatus. The effect of the proposed measure resides in that a region of the picture having a certain shading which lies between two fixed gradations, becomes recorded or reproduced by the mixture of points taken from both gradations. The mixing itself, in order that the recorded or reproduced picture portion would not become cloudy or to avoid the appearance of Moire therein, should be performed statistically. This is the reason why the frequency of the superimposed a.c. voltage must be non-harmonic with respect to the frequency of the scanning and recording or reproducing frequency (clock frequency).

In practice such requirement can be satisfied only with difficulties or not always with a certainty. For example, in the reproduction with varying scale, the scale variation is attained by varying the clock frequency. For the number of the possible scale variations there is a large number of clock frequencies available. Therefore, it can happen, that with respect to certain clock frequencies the requirement to obtain a non-harmonic cannot be satisfied. For each scale relation one must check if the superimposed a.c. voltage has any harmonic multiples with respect to the clock frequency and one should take care that such situation is prevented.

SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide an improved apparatus for the timed or pulsed conversion of an analog voltage into stepped voltages in which the harmonic relationship between the superimposed and the clock frequencies are safely and reliably eliminated.

According to the present invention the harmonic relations'hip between the clock and superimposed frequencies is eliminated by the continuous variation of the frequency of the superimposed a.c. voltage.

During the variation the a.c. voltage undergoing the variation can pass through several or a single time point at which it is for a short period in harmonic relationship with the clock frequency, however, even then the afore-mentioned danger is not present since the time period of such condition is smaller than the period of a drum rotation of the scanning and reproduction apparatus.

The invention is further characterized by the fact that the frequency of the superimposed voltage is varied statistically in an irregular fashion and the superimposed a.c. voltage is supplied by a generator the frequency determining elements of which are controlled or influenced by random generators.

A further advantageous feature of the present invention resides in that the superimposed a.c. voltage has a saw tooth shape having a tilt which varies irregularly in the statistical sense.

The preferred circuit arrangement for performing the method according to the present invention comprises a group of square wave generators having different frequencies and from the output of which by respective superimposition a train of irregular pulses is obtained and wherein a saw tooth generator having tilted edges and including RC-time members which can be selectively switched into the circuit and by which the tilt of the edge or the slope of the edge can be varied, further including switching means operated by the pulses of the random generator and with the help of which the RC- timing members can be selectively switched to cause fect directly following the time point at which the saw tooth voltage turns around or changes its sign.

BRIEF DESCRIPTION OF THE DRAWINGS DESCRIPTION OF THE PREFERRED EMBODIMENT The operation of the apparatus according to the present invention will be explained with reference to FIG. 1. As seen in FIG. 1 the circuit arrangement is subdivided into three groups A-C by the dashed lines. The part B is a self-oscillating generator which supplies an ac. voltage having a saw tooth-shaped curve. Between the conductor'l and the ground potential a capacitor 2 is provided. Conductor 1 leads also to a contact arm 3 of a relay l7 and to a terminal point 4 which represents the output of the generator B. The contact arm 3 closes in its illustrated position an energizing circuit which can be traced from the positive potential of a voltage source over a resistor 5, the contact arm 3, conductor 1, to the ungrounded plate of the capacitor 2. Through such energizing circuit a charging current flows which is determined by the magnitude of the resistor 5 and by the magnitude of the capacitor 2. The potential on the conductor 1 rises according to an exponential function.

The conductor 1 is further led to the input points 7 and 8 of a pair of comparators 9 and 10. To a second input 11 of the comparator 9 a small positive bias voltage is applied with the help of a potentiometer 12 which voltage serves as a reference voltage and, for example, can have a numerical value of +2V. To a second input 13 of the comparator a reference voltage having a similar value as mentioned above with respect to comparator 9 but having the opposite sign, that is, 2V is applied with the help of a potentiometer 14.

In the event the increasing voltage during the process of the charging will reach on the conductor 1 a value of +2V which is previously applied to the comparator 9, then the comparator 9 becomes actuated and through the conductor 15 it will energize the bistable relay 17 which in turn becomes actuated and will switch its contact arm 3 from the resistor 5, that is, from the positive side and connects the conductor 1 over the resistor 6 with the negative terminal of the voltage source illustrated only by its positive and negative terminals. There flows now from the capacitor 2 over the conductor 1, contact arm 3 and resistor 6 toward the minus terminal of the source a current which discharges the capacitor 2 and charges it to the opposite value, that is, negatively. The resistors 5 and 6 in this exemplary embodiment are similar, that is, equal in magnitude and, as a result, the time constant for the charging and discharging processes of capacitor 2 are also the same. The voltage of the conductor 1 will fall until it reaches the -2V value and ,over input 8 of the comparator 10 becomes effective. The comparator 10 as soon as becoming actuated, as mentioned in connection with comparator 9, delivers a pulse to the conductor 16 and switches the relay 17 into its original position. As a result, the contact arm 3 will change its position and connects the conductor 1 from the negative side of the voltage source to the positive side thereof and, as a result, a new charging process starts for the capacitor 2. The second period or process takes place similarly as the above described process. The voltage of the conductor 1 will vary in the remaining part of the process by the discharging and charging of the capacitor 2 between +2V. The resulting shape of the voltage curve is a saw-tooth shape with approximately straight edges since it has been assumed that the positive and negative source potentials are, for example, +1 2V and 1 2V and equal to each other and relatively large with respect to the applied bias or reference voltage +2V on terminal or input 11 and 2V on input 13 of the comparators 9 and 10 which on the exponential curve represent a relatively straight rise. The saw tooth shaped ac. voltage which is modified according to the present invention becomes available at the output terminal 4 and is delivered to a amplifier C. The saw tooth voltage will reach over a resistor 18 the input 19 of an operational amplifier 20. To the same input, that is, to input 19 of the operational amplifier 20, over a conductor 21 and over a resistor 22 the analog voltage to be digitalized is supplied, which according to the object of the present invention must be superimposed with the saw tooth voltage produced in stages A, B and D previously. The superimposition of these two voltages occurs in the operational amplifier 20 by addition. The sum voltage is delivered over the output conductor 23 for further processing, that is, to a analog-digital converter.

This saw tooth voltage must according to the present invention have a statistically irregularly variable frequency and, in certain special instances, it must have statistically irregularly variable tilts.

The slope of the curve edges is determined by the time constant, that is, by the magnitude of the capacitor 2 and by the charging and discharging resistors. In a simple fashion it is possible to vary such time constants by the insertion or removal of a parallel resistor to the capacitor 2. To this effect a resistor 24 is provided, as seen in FIG. 1, which at one end thereof is connected to the conductor 1. The other end of the resistor 24 is connected or connectable by means of a contact arm 25 to the zero or ground potential. When the resistor 24 is connected to the ground, it will lie parallel with respect to the capacitor 2. The contact arm 25 is operated by a relay 26 which is energized by voltage pulses which are delivered from the circuit section A over an input terminal 27. A changing of the steepness of the curve and the frequency of the saw tooth voltage produced by generator B is performed according to the preferred embodiment of the present invention at times which are independent from the very process of the production of the saw tooth voltages itself and occurs in such a manner that the tilts of the saw tooth voltage appear irregularly broken as will be described hereinafter.

From a random generator A over the output terminal 27 and the relay 26 square wave pulses are delivered the beginnings and the periods of which are irregular. For each pulse from the random generator A the relay 26 becomes energized and closes the contact arm 25. The operation itself is described with respect to FIG. 2. The curve 28 in FIG. 2a represents the saw tooth volt age as appears at terminal 4 when the resistor 24 is inoperative, that is, when the contact arm 25 is open. In the event the contact arm 25 is closed, then the resistor 24 becomes effective by being thrown into the circuit as described above. The charging current for the capacitor 2 will flow over resistor 5, while the discharging current will fiow over the resistor 6, and both, partly over the resistor 24, that is, if conductor 25 is closed, resistor 24 is in the circuit with the resistors 5 or 6. As a result, the charging and the discharging times become longer. The resulting saw tooth voltage will have a smaller frequency and is illustrated by the curve 29 in FIG. 2a, which assumes, of course, that the resistor 24 is in the circuit, which, however, is not the case at all times, as becomes apparent from the description of the effect of pulses 30.

Curve 30 in FIG. 2b shows a train of irregular pulses which through terminal point 27 reach the relay 26. At the times when the polarity of the pulses 30 is positive, then the relay 26 becomes energized. During the closed state of the relay 26 the generator B oscillates with a lower frequency represented by the wave form 29 while during the open state of the relay 26 the generator will oscillate at the higher frequency represented by the wave form of the curve 28 in FIG. 2a. At the time t the voltage in pulse train 30 is zero and the relay 26 is open, and as said above, the saw tooth voltage has a high frequency. Consequently, the wave portion 31 illustrated in FIG. and appearing on conductor ll has a slope corresponding to the slope of a wave form 28. At the time instant t the relay 26 is energized which obviously follows the risen state of the waveform 30 and consequently the contact arm becomes closed. The time constant of the charging and discharging of the capacitor 2 becomes larger and it follows the curve portions 32 the slopes of which correspond to those of the curve 29 at that time instant. At the time instant t the general direction of the curve of FIG. 2c changes without a change of the time constant associated therewith, since the changing direction means only the shift from capacitor charge to capacitor discharge. At the time instant i due to the fall of pulse the contact arm 3 again disconnects the parallel resistor 24. Looking at the waveform of FIG. 2c it is seen that this period is followed by curve portions 33 the form and character of which corresponds to the higher frequency curve 28 since resistor 24 is ineffective. At the time instants t t and t, the synchronization of the waveform 2c occurs in a manner as described with respect to the time instants t t and the curve portions 34, and 36 etc. are created and the slopes of which correspond with those of the curves 28 and 29 for reasons of controlling the time constants by the pulse shapes 30. Inasmuch as the intervals between the adjacent time instants can have different magnitudes and since the voltage variation of the resulting saw tooth wave is statistically irregular, a resultant waveform of FIG. 2c which includes the previously discussed curve portions 31-38 represents a distorted sawtooth voltage having a very unstable frequency. The frequency of such resultant saw tooth voltage varies as can be easily seen between the maximum frequency represented by the curve 28 the change-over to it occurring when the pulses at the terminal point 27 are negative or substantially negative and between the minimum frequency represented by the curve 29, than when the long positive pulses are present at the terminal point 27.

It might be desirable in certain circumstances to improve the bizarre form of the wave 40, but retaining its required properties, that is, that the saw tooth-shaped wave 40 should have tilts which vary statistically irregu larly. This requirement remains satisfied when the variations of the steepness or slope of the tilt do not fall randomly within the rise and fall times of pulses 30 but they occur only at the time instants when the rise and fall times interchange. These are the very time instants when the charging and the discharging periods themselves interchange. Referring such time instants to the curves 28 and 29, they correspond to their upper and lower peaks.

The steepness of the curve portions following such change depends from the nature of the potential which is present at the output 27 of the random generator A at the time instant of the change. Electric elements can be provided to insure that such potential can only are received from the output 15 and 16 of the comparators 10 or ill through the relay l7, reach the control input 48 of the flip-flop member 46.

The operation is as follows:

The pulses which have reached the inputs 43 and 44 of the flip-flop switching member 46 become stored but have no other effect. Consequently, at the output terminal 49 the polarity is retained which was left there by the previous operating phase. In the instant, however, when one of the two comparators 10 or lll become operative, through the conductor 15 or 16 and through the gate 47 a pulse will reach the control input 48 of the flip-flop member 46. At this instant the flip-flop member 46 follows the potential which lies at its inputs 43 and 44. If, for example, a positive potential is at the terminal point 411, then the potential on the conductor 49 is also positive, and will remain positive if it was positive during the previous phase of the operation. In the opposite case, the potential at the output terminal 49 will remain negative.

FIGS. 2d, 2e and 2f illustrate a variation which is the result of the above consideration. The random pulses represented by the curve 50 in FIG. 2d change similarly as the curve 30 in FIG. 2b, that is, they change irregularly between plus and minus. These potentials become stored in the flip-flop switching member 46 but they have yet no influence on the potential on the conductor 49. This will occur in the further process and will become represented by the curve 51 of FIG. 2e.

At the'time instant s both curves 50 and 51 are positive as determined by the previous occurrences in the circuit. At this time instant the voltage behavior of the produced alternating voltage, as seen in FIG. 2f,:

follows the steepness or slope of the curve 29 and is represented by the curve portion 53. At the time instant s the voltage of the curve 50 is equal to zero. The potential change becomes stored at the inputs 43 and 44 of the flip-flop switch 46. Curve 51, however, retains a positive potential since there is no switching pulse at the control input 48 yet. At the time instant s, the saw tooth generator B changes from the charging to the discharging state of capacitor 2. The pulse produced by the comparator 9 reaches over conductor 15, gate 47, the control input 48 of the flip-flop switch 46 which now will undergo a switching operation and will deliver a zero potential over the output terminal 49 to the terminal point 27. The relay 26 will become deenergized, that is, it will fall out and thereby opens up the contact arm 25. The subsequent discharge of the capacitor 2 occurs with a smaller time constant as controlled by the steeper slope of the curve 28. Onto the flat curve portion 53 a much steeper portion 54 is joined. At the time instant s;, the curve 51 has a zero potential and accordingly a similar potential. is at the terminal point 27. Relay 26 remains in its rest state and the contact arm 25 is open. The generator B further delivers a voltage which corresponds to the steepness characteristic of the curve 28 at this instant, however, at this time, in a rising sense. As a result, to the curve portion 54 a new curve portion 55 is joined. At the time instant s, the potential of the rectangular pulses will change to positive in accordance with the curve 50 at this instant. Accordingly the flip-flop switching member 46 will be preset as described above. It will, however, only at the time instant s switched by a pulse from the comparator 9 delivered through the conductor and gate 47 to its control input 48. Now a positive voltage will reach the terminal point 27 from the output terminal 49 and from the relay 26. Consequently, the contact arm 25 becomes closed and the resistor 24 will become thrown in parallel with respect to the capacitor 2 and will increase the time constant of the discharge of the capacitor 2. Tothe steep curve portion 55 now the curve portion 56 will be joined with a much smaller slope, that is, with a much flatter tilt, which corresponds to the low frequency curve 29.

At the time instants s s and s,,, the potential of the curve 50 will change, however, without causing a change of the slope of the curve portion 55. Only at the time instant s,, when the comparator l0 become energized with a pulse delivered over the conductor 16 and through the gate 47 to the control input 48 of the flipflop switching device 46 and, as a result, the flip-flop switch 46 will deliver a zero potential to the terminal point 27 and will cause the relay 26 to fall out, that is, to deenergize. The generator B operates again with a small time constant. As a result, to the curve portion 56 a curve portion 57 is joined which has much steeper slope.

The entire process now follows in a similar fashion, that is, steeper and flatter curve portions interchange in an irregular period sequence. As a result, there will be a saw tooth voltage produced which is represented by the curve 60 illustrated in FIG. 2f, the frequency of which changes suddenly and statistically irregularly.

Pulse generators which are capable of delivering statistically irregular pulses, such as, is the requirement to ,the saw tooth generator B, are known. It is also known to deliver such pulses by using a noise generator. It is a feature of the method according to the present invention to employ a random generator represented by the portion A in FIG. 1 in connection with the saw tooth generator B. The square wave generators 61-64, such as multivibrators, each of which oscillates at a constant frequency, deliver preferably symmetrical square pulses. The frequencies are then selected approximately according to their magnitude but in general they are undetermined and not stabilized. It is preferred, for example, to select the frequency of the first generator to have a magnitude of the lower saw tooth frequency, while the frequency of a second generator with about half of the frequency of the first generator and the frequency of the third generator with a frequency corresponding to a fourth of the first generator, etc. The outputs of the generators 61,62 and 63,64 are mixed exclusively by means of OR-gates 65 and 66 and the sum voltages delivered to the conductors 67 and 68 are mixed again by means of an exclusive OR-gate 69. At the output 70 of the random generator A an alternating voltage is obtained having rectangular pulses with a sufficiently random distribution. These pulses are then delivered directly to the terminal point 27 of the generator A if the waveform represented by FIG. 2c is required, on the other hand, if the waveform represented by FIG. 2f is required, then such pulses of the random generator A are delivered to the input terminal 41 of the storage device D in FIG. 1A. The output terminal 49 of the storage device D is connected with the input terminal 27.

A waveform delivered by the generator B is known from German laid open application DOS 1,772,367 as to be a saw tooth voltage. In the exemplary embodiment of the present invention it is shown that the rise and the fall of the waveform delivered by the generator B is symmetrical. However, without adversely effecting the effects of the present invention, the rise and the fall of the slopes can be made different and thereby to render the wave shapes produced by the generator B unsymmetrical. This can be achieved by simply changing the magnitude of the resistors 5 and 6 to be different with respect to each other.

In the exemplary circuit diagram illustrating the generator B, of course, the relay 17 having the contact arm 3 and relay 27 having the contact arm 25 can be replaced with equivalent electronic switching devices. The relays and contact arms have been selected to enable the easier and more clearer demonstration of the teachings of the present invention.

I wish it to be understood that I do not desire to be limited to the exact details of construction shown and described, for obvious modifications will occur to a person skilled in the art.

Having thus described the invention, what I claim as new and desire to be secured by Letters Patent, is as follows:

1. A method for improving the quantization of an analog voltage obtained through optical-electrical scanning of an image, wherein an alternating voltage is superimposed over the analog voltage to form a signal whose amplitude can subsequently be scanned at equal time intervals determined by a sampling frequency, said method characterized by the step of varying the frequency of the alternating voltage in a continuous fashion prior to superimposing it onto the analog voltage, such that said alternating voltage is disharmonic with respect to the sampling frequency.

2. The method as claimed in claim 1, wherein the frequency of the superimposed alternating voltage is varled in a statistically irregular fashion.

3. The method as claimed in claim 2, wherein said alternating voltage is produced in the form of a saw tooth waveform having slopes which vary statistically irregularly.

4. A circuit arrangement for producing an alternating voltage having a saw tooth waveform which waveform edges have statistically distributed irregular segments of different slopes, comprising a plurality of square wave generators each having a different frequency with respect to the other generators of said plurality, means for combining the output signals of said square wave generators for producing a pulse train signal having an irregular frequency of pulse occurrence and pulse width, a saw tooth voltage generator for producing a saw tooth voltage, said saw tooth wave generator comprising RC timing members, means for selectively switching said RC timing members to influence the slope of said saw tooth voltage, switching means for applying said pulse train signal for controlling the selective switching of said RC timing members to thereby vary the time constant of said saw tooth voltage generator, thereby producing a saw tooth voltage having a randomly varying slope and a statistically irregularly varying frequency at the output of said saw tooth generator.

5. The circuit arrangement as claimed in claim 4, wherein a storage means is provided coupled to the output of said plurality of square wave generators, means connecting a signal from said saw tooth voltage generators representing a change in sign of the saw tooth voltage to said storage means for adjusting said statistically irregular square wave pulse to coincide with the time instant of the change of said saw tooth voltage, and means connecting the input of said storage means to the output of said random square wave voltage generators.

6. The circuit arrangement as claimed in claim 5, wherein said storage means comprises a flip-flop generator having a set input, a reset input, a control input and an output, a the output of said random square wave voltage generators being connected directly to said setinput of said flip-flop generator and through an inverter gate to said reset input of said flip-flop generator, said control signal from said saw tooth generator being connected to said control input of said flip-flop generator. i k 

1. A method for improving the quantization of an analog voltage obtained through optical-electrical scanning of an image, wherein an alternating voltage is superimposed over the analog voltage to form a signal whose amplitude can subsequently be scanned at equal time intervals determined by a sampling frequency, said method characterized by the step of varying the frequency of the alternating voltage in a continuous fashion prior to superimposing it onto the analog voltage, such that Said alternating voltage is disharmonic with respect to the sampling frequency.
 2. The method as claimed in claim 1, wherein the frequency of the superimposed alternating voltage is varied in a statistically irregular fashion.
 3. The method as claimed in claim 2, wherein said alternating voltage is produced in the form of a saw tooth waveform having slopes which vary statistically irregularly.
 4. A circuit arrangement for producing an alternating voltage having a saw tooth waveform which waveform edges have statistically distributed irregular segments of different slopes, comprising a plurality of square wave generators each having a different frequency with respect to the other generators of said plurality, means for combining the output signals of said square wave generators for producing a pulse train signal having an irregular frequency of pulse occurrence and pulse width, a saw tooth voltage generator for producing a saw tooth voltage, said saw tooth wave generator comprising RC timing members, means for selectively switching said RC timing members to influence the slope of said saw tooth voltage, switching means for applying said pulse train signal for controlling the selective switching of said RC timing members to thereby vary the time constant of said saw tooth voltage generator, thereby producing a saw tooth voltage having a randomly varying slope and a statistically irregularly varying frequency at the output of said saw tooth generator.
 5. The circuit arrangement as claimed in claim 4, wherein a storage means is provided coupled to the output of said plurality of square wave generators, means connecting a signal from said saw tooth voltage generators representing a change in sign of the saw tooth voltage to said storage means for adjusting said statistically irregular square wave pulse to coincide with the time instant of the change of said saw tooth voltage, and means connecting the input of said storage means to the output of said random square wave voltage generators.
 6. The circuit arrangement as claimed in claim 5, wherein said storage means comprises a flip-flop generator having a set input, a reset input, a control input and an output, a the output of said random square wave voltage generators being connected directly to said set-input of said flip-flop generator and through an inverter gate to said reset input of said flip-flop generator, said control signal from said saw tooth generator being connected to said control input of said flip-flop generator. 